Referring to FIG. 1, segments for the primary mirror of the next generation of large ground based telescopes (such as the Thirty Meter Telescope) are currently designed as approximate hexagonal shapes close to 1.44 meter corner to corner and approximately 45 mm thick, with aspheric surface shapes. They are supported on a multi-point mount to minimize gravity induced deformations.
In use, the primary mirror may be tilted between zenith (i.e., pointing straight up) to approximately 80 degrees from vertical. The element mount design is configured to minimize the change in shape of the optical surface. For example, FIG. 2 shows the predicted shape change of an element of the Thirty Meter Telescope primary mirror when the pointing direction changes 80 degrees. There is an active bender designed into the system. The response of the mirror to forces applied by the bender can be predicted. Validation of the mount design and the predicted behavior of the bender may require surface figure measurement with less than, e.g., 15 nm root-mean-square (rms) uncertainty.
The surface profiles of the mirrors can be measured using a large test tower having a reference optic having a reference surface with a size that corresponds to the size of the surface of the part being tested. Measuring deformations with change in the gravity vector may involve, e.g., changing the orientations of the reference optic and the part under test.